1. Field
This invention relates to an uninterruptible power supply, and more particularly to an uninterruptible power supply that adjusts the monitored/perceived value of the amplitude and/or frequency of the input voltage so that the uninterruptible power supply accepts the unstable voltage and/or frequency, thereby preventing the uninterruptible power supply to switch back to the battery mode during a transfer time between the battery mode and the AC mode.
2. Description of Related Art
The following description sets forth the inventors' knowledge of related art and problems therein and should not be construed as an admission of knowledge in the prior art.
As illustrated in FIG. 1, an uninterruptible power supply (UPS) has a generator 8 that is provided as an alternate source of AC power supply in an event of a power failure. The switching between commercial power 9 and a generator 8 is performed by a switch 10. The UPS system also constitutes a battery thyristor 1, a main capacitor module 2, a battery module 3, an input filter circuit 4, an input converter module 5, an output inverter module 6 and an output filter module 7.
As shown in FIG. 2(A), during switching time from commercial power 9 to generator power 8, the UPS supplies power to the load using the battery module 3. Normally, during this switching time, the voltage across the main capacitor module 2 is boosted up by a battery package using a voltage boost up circuit.
However, as illustrated in FIGS. 2(A) and (B), when the UPS is switched from the battery backup mode to the generator mode, the battery thyristor (THY) 1 turns off and the mode is transferred to AC input mode. During this transfer, which may take 3 to 10 cycles, the DC voltage across the main capacitor module 2 drops because of the energy consumed in capacitors as shown by the exponential decay in the graph of FIGS. 2(A) and (B) (NVD=Target Voltage Drop across the main capacitor module 2).
The calculation for DC voltage drop across the equivalent circuit of the main capacitor module 2 in view of FIGS. 2(A) and 2(B) is as follows:
For example: UPS output 10 KVA                Output Power Factor (p.f.)=0.85        DC/AC efficiency=85%        Period (3 cycles)=50 ms between Battery and        AC modeEnergy consumption(J)=UPS output(KVA)*output p.f.*1/DC-AC Efficiency*period  (1)Energy consumption(J)=10*103*0.85*1/0.85*50×10−3         
Energy consumption (Ec)=500 JoulesThe Energy storage at main capacitor=½*C*V2  (2)For equivalent capacitance in main capacitor module 2=8400 μF
C=8400 micro farads, and
Vo=770 Volts½*C(Vo2−V12)=Ec  (3)Therefore,
                                                                        V                ⁢                                                                  ⁢                1                            =                            ⁢                                                                    Vo                    2                                    -                                                            2                      ⁢                      Ec                                        C                                                                                                                          =                            ⁢                                                                                          770                      2                                        -                                                                  2                        ×                        500                                                                    8400                        ×                                                  10                                                      -                            6                                                                                                                                              =                                  688                  ⁢                                                                          ⁢                  V                                                                                        (        4        )            Therefore, ΔV=Vo−V1=770−688=82 Volts drop
Therefore, when the generator input voltage is applied to the UPS, a large inrush current flows due to the voltage difference between the capacitor bank's actual voltage and target voltage, which causes the voltage from the generator to drift causing unstable condition in generator operation as shown by the graph in FIGS. 2(A) and (B).
In other words, during switching power from battery module 3 to the generator 8, a high current flows through the UPS due to the voltage drop in the DC bus. Especially, in a case of generator connection to AC line, this current may become large due to oscillations caused by the generator's internal impedance (mostly inductive) and the impedance of input filtering capacitor. As a result of the large oscillatory current, the output voltage of the generator becomes unstable. When the UPS detects this unstable voltage and/or frequency, it moves the UPS to Battery backup mode again. In the case where inrush current is too large, the problem of repeated switching between the generator mode and the battery backup mode occurs thereby causing battery to burn up additional power.